In order to monitor tire inside information such as an internal pressure and a temperature, there have been heretofore performed attachment of a sensor device to a well part of a rim, detection of the tire inside information by the sensor device, and transmission of the detected results to a receiver outside the tire by use of radio waves.
Since such a sensor device for tire is driven by a built-in battery, it is desired to lower a frequency of detection or transmission during stopping, in order to extend life of the battery. Thus, a centrifugal switch for detecting a traveling state is provided in the sensor device, and, based on the detected results of the centrifugal switch, an operation of a sensor or a transmitter is switched between a high frequency traveling mode and a low frequency non-traveling mode (for example, Japanese patent application Kohyo publication No. Hei 10(1998)-508264).
However, although the centrifugal switch detects the traveling state when predetermined centrifugal force occurs, the centrifugal force acting on the centrifugal switch changes depending on a wheel structure and an attachment position of the sensor device even at the same traveling speed. In some cases, there is a problem that the switch is not operated until the traveling speed is considerably increased. For example, in attaching the sensor device to the well part of the rim, if a rim diameter is the same and a tire outside diameter is different, the larger the tire outside diameter is, the less the centrifugal force is. Moreover, in attaching the sensor device to the well part of the rim, if the tire outside diameter is the same and the rim diameter is different, the larger the rim diameter is, the more the centrifugal force is. Since the centrifugal switch built into the conventional sensor device has no mechanism to adjust an operating speed, sensitivity thereof cannot be adjusted according to the wheel structure and the attachment position of the sensor device.